Among known mobile communication systems is a cellular system in which a wireless network composed of at least a plurality of base stations performs communication with a large number of mobile stations lying in one or more cells formed by the respective base stations. In this cellular system, the same frequency carriers (hereinafter, referred to as “carriers”) are used in a plurality of cells simultaneously so that large amounts of user information can be communicated between the wireless network and the large number of mobile stations.
For this cellular system, such a method as shown in FIG. 9 has been known in order to make the same carriers simultaneously usable in as many cells as possible. According to this method, each cell's coverage is sectioned into two areas, an area closer to the base station (hereinafter, referred to “intracell inner area”) and an area farther from the base station (hereinafter, referred to as “intracell outer area”). For the intracell outer areas, predetermined carriers are divided into three groups (f1, f2, f3), for example. The carriers of each group are only assigned to every three cells so as not to be used in adjoining cells simultaneously. Meanwhile, other carriers (F) are assigned to the intracell inner areas.
This method of assigning and using different carriers in the intracell inner areas and intracell outer areas has been described, for example, in Patent Document 1.
To practice this method of using carriers, the base stations transmit a common pilot signal with predetermined transmission power in the respective cells. Mobile stations receive the common pilot signal from a plurality of neighboring base stations, and measure the received power. The mobile stations then select a cell of the highest received power as their own cell to perform communication with, and a cell of the second highest common pilot signal as an adjoining cell. If the ratio between the received power of the common pilot signal of the own cell and that of the adjoining cell is higher than or equal to a predetermined threshold (hereinafter, referred to as “intracell-inner-area threshold”), the carriers for the intracell inner area are used. If not, the carriers for the intracell outer area are used. In another possible method, instead of using the ratio between the received powers of the common pilot signal, the carriers for the intracell inner area may be used if the reception quality of the common pilot from the own cell (such as the received power of the common pilot signal and the ratio between the received power and interference signal power) is higher than or equal to a predetermined threshold. If not, the carriers for the intracell outer area are used.
As above, in the cellular system where a plurality of carriers are simultaneously used in a plurality of cells, the spectral efficiency of the entire system improves and the traffic capacity increases when as many carriers as possible are made available at high use rates in as many cells as possible.
Patent Document 1: JP 6-311089-A